1100 Anal. Chem. 1992, 64, 1100-1105

Neutron Activation Analysis for Reference Determination of the Implantation Dose of Cobalt Ions

Rainer P. H. Garten* Mar-Planck-Institut far Metallforschung, Laboratorium fur Reinststoffanalytik, Bunsen-Kirchhoff-Strasse 13, 0-4600 Dortmund 1, Germany

Henning Bubert Institut fiir Spektrochemie und angewandte Spektroskopie, Bunsen-Kirchhoff-Strasse 11,D-4600 Dortmund 1, Germany

Leopold Palmetshofer Institut fiir Experimentalphysik, Johannes-Kepler- Uniuersitat, A-4040 Linz, Austria

RellaMe quantlficatlon of methods for surface and depth pro- flllng analysis depends on the avallablllty of reference mate- rlak for callbratbn. Slnce predskn of depth prolillng analyds procedures reaches the 1-2% level, the elemental contents In reference materials should be characterized wlth according accuracy. As an example, we prepared depth proflllng ref- erence materlals by cobatl Ion lmplantatlon at an Ion energy of SO0 keV Into n-type rlllcon; the Implanted Co dose was monltored by Ion current measurement (ICM). The total Im- planted Co Ion doses wore determlned by Instrumental neu- tron actlvatlon analysls (NAA) In the standard comparkon mode, withln a dynamic range of nearly 5 decades. The uncertalnty amounted to less than 1.5 % . It was found that the relathre Mas was (10 f S)% of the lmplantatlon dose as measured by ICM In the dose range from 10l2 to lo1' Co lons/cm2. Sources of error (beam spreadlng, mlsallgnment) can be corrected lor In thls way. The advantages of this approach with slmllar samples of thls type Is outllned. The detect1011 IWI was 5 x loe Co 10ne/cm~. It can be hproved to lower than 10' lons/cm2 for 27 elements to be Implanted In hlgh-purity materials.

INTRODUCTION The continuing trend of miniaturization in technology

challenges and drives the development in surface and thin-fii depth profiling analysis methods.14 The prominent methods Auger electron spectrometry (AES), X-ray photoelectron spectrometry (XPS), secondary ion mass spectrometry (SIMS), ion scattering spectrometry (ISS), and Rutherford backscattering spectrometry (RBS) are generally capable of a relative precision on the 1-2% level,1~510 under favorable conditions. Reliable quantification, however, of these in- strumental spectroscopic methods depends on the use of calibration samples that can be prepared with a well-defined near-surface composition and distribution by means of ion implantation.

Definite absolute calibration of such materials requires determination of the total implanted ion dose density D, abbreviated as dose D (this term is more widely accepted" than the physically more exact term fluence; see ref 12, p 780), by an independent means, i.e., by precise and accurate de- termination of the elemental contenti This applies to the case of the so-called integration method (see refs 13,14, cf., refs

* Corresponding author.

12,15), and also to the general case of calibration of the molar fraction scale and of the corresponding sensitivity factors16 by comparison of signal intensities. To fully exploit the ca- pabilities of the depth profiling methods, the total implanted dose should be determined at an accuracy levePJ7J8 of 1-2% accordingly.

Ion implantation is further used for the modification of surface layers and the preparation of special subsurface layers in materials.l9a In particular, implantation of metal ions (e.g. Co, Ta, Mo, Ti, Ni) in semiconductor devices is used in very large scale integration (VLSI) technology to form conductive silicide lines as interconnects!p21-B Buried Co silicide layers of this type have been characterized e l s e ~ h e r e . ~ ~ * ~ ~ - ~ ~

In matrices like high-purity target materials, instrumental neutron activation analysis (INAA) is a powerful methodm with high sensitivity for a number of elements that are to be quantified after their ion implantation. Basically, INAA is a very simple two-stage process consisting of (1) irradiation and (2) separated measurement. Matrix effects, especially of chemical origin, are often very low or negligible in INAA. Hence, with a number of high-purity materials, including metals, semiconductors, and ceramics, freedom from bias can be achieved by careful control of the identifiable sources of uncertainty (refs 31-34 and 35 pp 445-502), viz. (1) sample contamination, blanks, and homogeneity, (2) contamination and losses of volatile compound due to heating during irra- diation, and contamination due to recoil of fission fragments from wrapping material, (3) preparation of standards and calibration procedure, (4) differences between the effective neutron flux densities that are incident onto samples and standards, (5) interfering nuclear reactions yielding the in- dicator nuclide from other parent elements in the sample, (6) counting geometry, dead time, and pile-up losses, attenuation of y-rays, and (7) counting statistics.

For example, Table I givea a survey of the most appropriate nuclides and the relevant figures of merit for a silicon matrix. Furthermore, INAA is generally nonconsumptive and causes only minor or negligible damage to the samples considered here. Samples of very low element content (including any possible additional impurities) can be used subsequently in any other analytical procedure. Samples of moderate element contents require cooling during about 10 half-lives, regarding also activity produced by interfering reactions from sample constituents, e.g. %Na activity from primary interference re- action 27Al(n,a)aNa in Al-based matrices. Samples of higher activity have to be considered definitely as nuclear waste, but those samples are needed only in small amounts due to the high sensitivity of NAA.

0003-2700/92/0364-1100$03.00/0 0 1992 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 64, NO. 10, MAY 15, 1992 1101

Table I. Survey of Most Appropriate Nuclidesa (See Discussion: Detection Limit) for Combined Ion ImplantatiodNAA Calibration in High-Purity Silicon and Relevant Figures of Merits

element

Na Mg Al s c Mn Fe c o Ni c u

Zn Ga Ge AS Se Br Kr Rb Sr Y Zr

Mo Ru

Pd

Ag Cd In Sn S b Te Xe

cs La Ce

Pr Nd

Sm Eu

Gd Tb DY Ho Er Tm Yb Lu

Hf Ta W Re os Ir Pt

Au Hg Th U

implanted nuclide

Na-23 Mg-24 Al-27 sc-45 Mn-55 Fe-56

Ni-58 CO-59

CU-63

Zn-64 Ga-71 Ge-76

Se-82 Br-81 Kr-84 Rb-85 Sr-86 Y-89 Zr-94 Zr-96

AS-75

Mo-98 Ru-102 Ru-104 Pd-108

Pd-110 Ag-109 Cd-114 In-115 Sn-116

Te-130 Xe-132 Xe-130

La-139 Ce-140 Ce-142 Pr-141

Sb-123

CS-133

Nd-146 Nd-150

Eu-151

Gd-158 Tb-159 Dy-164 HO-165

Sm-152

Er-170 Tm-169 Yb-174 Lu-175 Lu-176 Hf-180

W-186 Re-187 OS-192 os-190 Ir-191 Pt-196

Au-197 Hg-202 Th-232 u-238

Ta-181

natural relative

abundance

1.0 0.79 1.00 1.00 1.00 0.92 1.00 0.68 0.69

0.49 0.40 0.077 1.00 0.090 0.49 0.57 0.72 0.099 1.00 0.17 0.028 0.24 0.32 0.19 0.27

0.12 0.48 0.29 0.96 0.14 0.43 0.35 0.27 0.041 1.00 1.00 0.88 0.11 1.00 0.17 0.056 0.27 0.48

0.25 1.00 0.28 1.00 0.15 1.00 0.32 0.97 0.026 0.35 1.00 0.29 0.63 0.41 0.26 0.37 0.25

1.00 0.30 1.00 0.99

y-radiating activation product

Na-24 Na-24 Na-24

Mn-56 Mn-56

SC-46

CO-60 CO-58 CU-64

Zn-65 Ga-72 Ge-77

Br-83 Br-82 Kr-85m

Sr-87m Y-90m Zr-95 Zr-97

AS-76

Rb-86

Mo-99 Ru-103 Rh-105 Pd-109

Ag-111

Cd-115 Ag-llOm

In-115m Sn-ll7m Sb-124 5-131 Xe-133 Xe-l3lm

La-140 Ce-141 Ce-143 Pr-142

Pm-151 Sm-153 Eu-152ml

(3-134

Nd-147

Eu-152 Gd-159 Tb-160 Dy-165 HO-166 Er-171 Tm-170

Lu-176m Yb-175

Lu-177 Hf-181 Ta-182 W-187 Re-188 OS-193 os-191 Ir-192 Pt-197

Au-198 Hg-203 Pa-233 Np-239

overall sensitivity of

half-life 7 INAAb

15 h 15 h 15 h 84 d 2.6 h 2.6 h 5.3 y 71 d 13 h

244 d 14 h 11 h 26 h 2.4 h 36 h 4.5 h 19 d 2.8 h 3.2 h 65 d 17 h 66 h 40 d 36 h 13 h

7.5 d 250 d 53 h 4.5 h 14 d 60 d 8.0 d 5.3 d 12 d 2.1 y 1.7 d 32 d 1.4 d 19 h 11 d 28 h 46 h 9.3 h 13 Y 19 h 72 d 2.4 h 27 h 7.5 h 130 d 4.2 d 3.7 h 6.7 d 42 d 114 d 1.0 d 17 h 30 h 15 d 74 d 18 h

2.7 d 47 d 27 d 2.4 d

0.48 0.00022d 0.00012d 3.9 0.38 0.00013d*' 0.23 0.0032d3' 0.00048

(0.58)c 0.00092 1.4 0.038e 0.89 0.00021e 1.9 0.020e 0.0074 0.021e 0.00012 0.0039 0.011 0.16 0.14 0.072 0.0014

(0.059)c 0.0026s 0.13 0.089 0.00lOd*~ 0.0029 0.56 0.093' 0.85' 3.9" 0.31 2.6 0.067 0.17 0.041 0.050 0.061

36 120

7.5 0.055 0.94 0.46' 1.5 1.3 0.17 4.5 0.08

5.8 0.78

150e

14 17 0.078 0.32

0.023 (0.094)c 0.24 0.29 0.35 0.28

280

detection limit in log ions implanted per omz

this workf other authors"

2 w

1.4

6

308 50 108 408 308

358

240 708

1000

38

0.18

30

2008

20 16 138

5000 30 15 15 0.3 2

10 90 308 208 50 78

20 0.088 0.09

2008 2

158 128 8 7

0.7 2.6 28 0.68

100 12 0.013 3

0.25% 10 5

46 .w

3 w

0.03' 3.E4k

0.6' 404

3 d 9' 0.6'

50k 0 .9

0.2k

l j

lok 3' 0.4k

3k 0.5k 0.7k

0.u' 0.5k

0.2k 0.03k 0.5"

2k 0.5k

0.002k 0.09 0.04k

0.05'

1,5' 3' 1.5' 0.005k

0.0004k 0.03k 0.2' 0.0~3~ 0.009k

0.02k 0.0006'

0.0003i* 0.06k 0 . 0 6 k 0.1k

"Boundary conditions for selection: a 2 0.1, 7 > 2 h, sensitivity s/tm 2 lo4 counts s-l per 10l2 atoms. Elements prominent in ion implantation work are highlighted by boldfaced lettering. bEstimated figures of merit: sensitivities per counting interval s/tm quoted to read roughly as counts per s and 10l2 implanted atoms, assuming a cooling period less than 12 h, under the experimental conditions as applied in this work. 'Numbers in parentheses referencing to y-lines which are frequently subject to interferences by other nuclides. dSensitivities for (n,p) and (n,n') reactions are based on irradiation a t an epithermal flux of 1.5 X 1Ol2 ns-l cm-l, available at channel BE20 of the same reactor FRJ-2 (cf. refs 36,37). OBased on nuclear data taken from refs 38, 39. fDetection limits as measured from blanks after a cooling period of 10 days. #Extrapolated detection limita regarding radioactive decay, assuming a cooling period of more than 15 h, by scaling from blank measurement after 4 days. hoptimum detection limits, recalculated from bulk data obtained from ultra-hieh-ouritv material (nonimdanted) given by. 'Verheijke et al.N 'B6ttger et al.41 kVerheijke et al.42

1102 ANALYTICAL CHEMISTRY, VOL. 84, NO. 10, MAY 15, 1992

Several Co ion implants in high-purity silicon wafers (listed in Table 111) were analyzed for the total implanted ion dose D by INAA in this work, using the standard comparison method. The results were compared with the results from initial determination by integration of the implanted ion current at the accelerator during the preparation of the sam- ples, and from determination of the dose by X-ray fluorescence analysis (XRFA), flame atomic absorption spectrometry (FAAS), and RBS, as published r e ~ e n t 1 y . l ~ ~ ~ ~

EXPERIMENTAL SECTION Specimen Processing. The implantation by a 350-kV low-

current implanter has been described e1~ewhere.l~ Briefly, the vacuum in the target station was about 2 X lo4 Pa during the implantation. Wafers of n-type silicon (Wacker Chemitronic, Burghausen/D), (100) oriented, measuring 30 X 30 mm2 in size, were implanted with single-charged 5sC0 ions of kinetic energy of 300 keV at room temperature. To avoid channeling, an azi- muthal angle of 14" and a polar angle of 7.5O were chosen. The implantation doses DICM (see Table I11 column 2 and ref 15) were determined by integration from ion current monitoring (ICM) using an arrangement4 of 3 conventional Faraday cups." After implantation, the samples were cut into piece for depth and calibration analysis described in this paper.

Reagents. Handling of samples, standards, and Al strips was throughout done using thoroughly precleaned tweezers and cutting tools. Double distilled water from a quartz glass device, and pro analysis (Merck, Darmstadt/D) acids and solvents were used throughout the procedures. For wrapping of samples and standards, aluminum foil (high-purity Kryal OZ, Vereinigte Aluminiumwerke, Cologne/D, Co content of 5 ng/g 4 0.3 X 10l2 Co atoms/cm2 at 24-pm thickness, U content of 50 ng/g; or, as far as appropriate for the standards of higher Co contents, technical purity degree Al foil, Melitta, Minden/D, Co content of 0.6 pg/g 4 16 X 10l2 Co atoms/cm2 at 10-pm thickness) was covered with clean paper and cut into strips of suitable size (ca. 20 X 40 mm2; ca. 70 mg or 30 mg each) using a paper-cutting machine. Strips were stored in a polystyrene box until use. All fused silica flasks, pipets, and vials were cleaned by steaming for 6 h using HN03 and for 2 h using H20, dried at 105 "C.

Sample Preparation prior to Analysis. Samples, nonim- planted Si blank wafers of ca. 15 X 15 mm2 (8 X 8 mm2 of samples Co-A2 and Co-A3), and standards were wrapped into A1 foil. Samples were then sandwiched between prepared standards of adapted Co contents and placed in a precleaned vial made of high-purity fused silica (Suprasil of Hereaus, Hanau/D): The vial was sealed by means of an oxyhydrogen flame using a burner made of fused-silica. In this way, maximum care was taken to avoid any contamination from the sample preparation ~ t e p . ~ , ~ '

Standards. The standard comparison method was used for Co determination by NAA (see refs 35, 36). Standards were prepared from cobalt spectrometric standard solution (SRM 3113, NIST, Gaithersburg, MD/USA) 10 mg/mL, freshly diluted to 500 pg Co/mL, 50 pg Co/mL, 5 pg Co/mL, and 0.4 pg Co/mL in 10% HNOP All solutions were prepared by gravimetric control using fused silica flasks and pipets, from HN03 freshly distilled by subboiling distillation.

Co standards were prepared on a clean-bench (US standard 100). Filter paper coupons of 15 X 15 mm2 and 8 X 8 mm2 were cut from ashless paper blue ribbon (Schleicher and Schiill, Dassel/D). For cutting, the filters were sandwiched between clean fiiter papers. These covering papers were discarded. Filter paper coupons were stored in polypropylene bottles until use. Another set of standards was prepared using graphite rods, spectroscopy grade, 6.15-mm diameter 1 X 1 mm (RW-1) grade, Ringsdorff, Bad Godesberg/D), in the same way.

All standard solutions were dosed using a microburette as- sembly (Beckmann Instruments/USA) with a measured relative uncertainty of less than 0.5-1.1% in the volume range 10 to 2 pL. (Precisions and uncertainties are throughout quoted as 1s standard deviations obtained by error propagation from all contributing sources of error.) The solutions were manually blotted to be dispensed uniformly over the filter coupon area (graphite rod, resp), soaked up, and allowed to dry in the airstream of the clean bench under mild radiation heating up to 60 "C by a bulb. Filter

Table 11. Nominal Co Mass m of the Prepared Standards and Mean Relative Signal Intensities y To Establish the Calibration Function and RSD of the Intensities

nominal Co mass m (ng)

6000 2899 lo00 301 100.3 50.5 20.2 5.05 1.02 0.203

prepn probable relative

error (%)

0.9 0.6 1.1 0.6 1.4 0.6 0.6 1.7 1.1 3.2

no. of replicas,

n

4 6 8 5 7 3 5 5 4 4

Y

5985 2908 990 299 100.6 50.1 19.9 5.04 1.03 0.203

RSD (%)

0.4 0.6 3.4 1.2 2.5 0.6 2.5 8.9 8.0

11.9

coupons (rods) were then wrapped in Al foil, and identification nunberings were written on these Al foils using a hard pencil. After irradiation, standards were left in its Al cover and packed separately in polyethene bags. Solutions were dosed to 0.5,1,2, 4,6, and 10 pL to prepare the standards listed in Table 11.

Standards of about 6 pg of Co were prepared by cutting 5-mg pieces from standard reference material 953 (NIST, Gaithersburg, MD/USA) "neutron density monitor wire (cobalt in aluminum)". Blanks on filter coupons and graphite rods were prepared by dosing 7 pL of the HN03 used for dilution, according to the standards preparation procedure. Element contents and probable errors of calibration samples are given in Table 11.

Irradiation. The vial containing samples and standards was irradiated in the research reactor FEW-2 (23 MW) at Jtilich, channel BE 4, at a thermal flux of (9.04 * 0.05) X 10l2 n cme2 s-l (center of vial; epithermal flux of (7.2 * 0.3) X los n cm-2 s-l, nominal temperature of 90 "C) for a time duration ti, of 20 days.

Measurements. During handling of irradiated samples, the highest radiation dose after a cooling period of 10 days appeared with the standards of 6 pg of Co. At a working distance of 30 cm, the highest radiation dose was leas than 2 p Sv h-l, and it could be reduced to less than 0.1 p Sv h-l by using a shielding wall of lead of 5-cm thickness.

A high-resolution y-spectrometer was used, consisting of a coaxial intrinsic germanium detector 157 cm3 (PGT IGC 40, Princeton Gamma Tech, Wiesbaden/D), 8K multichannel ana- lyzer (ND 66, Nuclear Data, Schaumburg, IL/USA), on-line spectrum analysis and data handling system (ND neutron acti- vation analysis package on VAX/VMS). System resolution was 1.9 keV at 1332 keV, at a peak-to-Compton ratio of 701; efficiency was 40% relative to a 3- X 3-in. NaJ(T1) detector. Detector shielding: 5 cm of lead with inner lining of Cd/Cu/Al/PE at a minimum distance of 17 cm from detector and sample.

Standards and samples were covered by thin PE foils and were either (1) pressed between two lucite sheets of 1-mm thickness and fiied at a distance of 150 i 0.1 mm to the detector by means of a lucite mount, or (2) gently pressed to the center of the detector's end cap by means of a lucite mount. In this way the total countrate was adjusted not to exceed 4000 counts/s.

All samples and standards were measured in duplicate, after a cooling period of 10 days and 40 days, respectively, for measuring times t , from 10 min to 20 h, to obtain statistical counting errors of less than 0.2% with Co masses m I 50 ng (counting error of less than 0.3% with m I 1 ng of Co and of less than 0.5% with m 5 0.2 ng of Co). The sum of y-lines 1172 and 1332 keV emitted from 6oCo (half-life = 5.272 y) was used for evaluation.

Control of Surface Contamination. To remove any posaible contamination from the surfaces (cf. ref 42) of blanks, and of samples CeL1 and CeL2 (i.e., those of low Co content), an etching digestion was applied, which was adjusted to remove a layer of 1.1 0.5 pg cm-2 (44.7 nm Si) thickness from nonimplanted Si wafers. Within this thin surface layer, the relative Co amount is less than 0.3% of the total implanted Co dose D (for implan- tation profiles produced by 300 keV Co+ and at total doses below ioi7 Co ions/cm2).

These samples were digested, using ultrasonic agitation, in the following sequence: (a) degrease in toluene at 70 O C for 10 min,

ANALYTICAL CHEMISTRY, VOL. 64, NO. 10, MAY 15, 1992 1103

Table 111. Results from Individual Implanted Samples As Measured by INAA (Column 5), Its Relative Standard Deviation RSD, and the Dose D Deduced from Measurements (Column 7)"

nominal dose, relative DICM Co mass by Co ion dose systematic

sample code (Co/cm2) mass (g) area (cm2) INAA (ng) RSD (%) D (Co/cmz) deviation (%)

blank A blank B blank C blank D co-L1 co-L2 C O - A ~

0.2515 0.2209 0.2563 0.1884

1 x 10'2 0.2739 1 x 1014 0.2663 1.6 X 10l6 0.06964 5 x 10'6 0.07447

2.091 1.836 2.131 1.566 2.277 2.214 0.5789 0.6190

<mL <mL <mL

1.5 x 10-3 15. 9.8 x 109 0.199 5.1 8.91 X 10" -10.9b f 5

19.56 1.4 9.02 x 1013 -9.8 781 1.3 1.379 X 10l6 -13.8

2814 1.2 4.645 X 10l6 -7.1

Column 8 gives the relative systematic deviation of ion current measurements ICM: ( D N ~ - DICM)/DICM X 100, as derived from NAA results. Uncertain due to analvtical error.

(b) dry at 25 "C in air, weigh on an ultramicrobalance, (c) digest in concentrated HN03 at 30 "C for 10 min, (d) rinse with water, wash in water at 25 OC for 1 min, wash in acetone, dry at 25 OC in air, (e) etch in aqueous concentrated HF, at 30 "C for 10 min (Caution: HF solution is extremely corrosive and appropriate safety procedures must be employed to avoid its unhealthy ac- tion!).

The samples were further treated in sequence (d), (c), (d), (e), (d) and weighed. Sample CeL2 was only treated once, in sequence (a), (b), (c), (d), (e), (d), since Si erosion was significantly higher as compared to the Si blanks. Following this digestion, samples were remeasured by y-spectrometry for t, = 20 h each.

Determination of Sample Surface Area. The areas of the activated samples were calculated from samples massea. The masa per unit area of the Si wafers used was determined to be 0.1203 f 0.0004 g cm-2 via optical area determination. Area determi- nations of individual samples (see Table 111) are uncertain to f0.4%.

Corrections. Data were corrected for minor pulse pile-up and dead time losses using a reference pulser (cf. ref 31), as well as for radioactive decay and were normalized (with an uncertainty of 0.4%) to the standard distance of 150 mm between sample and detector, based on measurements of a multiple set of Boco samples.

To take into account the axial gradient of neutron flux 9 and neutron dose 9 ti, in the irradiation ampoule, an average linear flux gradient of (A@/Al)/@ = (3.38 f 0.04)% per cm was estimated by linear regression of activity measurements, based on six flux monitor wires of Zr (MARZ grade, Alfa inorganics, Karlsruhe/D) and four monitor wires of Al/Co alloy (SRM 3113, NIST, Gaithersburg, MD/USA). This flux gradient was applied to correct measured activity data of both standards and samples.

As interfering reactions, we considered the fast neutron-induced reactions BONi(n,p)BOCo and 63Cu(n,a)BOCo. With the highly thermalized flux applied here, interference levels were estimated to 4 X at/at for 63Cu (based on data compiled in ref 36). Since any initial Ni and Cu content would have contributed to the blanks equally, this source of bias was considered to be insignificant.

Considering contamination of the sample during irradiation due to recoil of fission fragments, the uranium contamination of the wrapping material has to be controlled when determining %Zr, 98M~, %e, l3@I'e, Ru, and several rare earth nuclides. This contamination is worst for =Zr in a highly thermalized neutron flux, as applied in this work. A rough estimate yields a simulated dose of about 3 X 1015 atoms/cm2 of *Zr in Si and less than l O I 3 atoms/cm2 of all other nuclei, recoiled from the Al foil used. Samples have to be sandwiched between pure silicon for determination of those interfered nuclei. Determination of Co is not affected.

Differences in neutron self-shielding and y-ray attenuation in samples and standards were estimated to be negligible, so no corrections were applied.

RESULTS Calibration. Co contamination levels per calibration blank

sample were found to 170 f 25 pg of Co per filter paper coupon 15 X 15 mm2, and 40 f 15 pg of Co per graphite rod, including the respective Al envelopes. These blank levels were corrected

at/at for BONi and to 5 X

for in the low-content standards. For standards prepared from filter papers and graphite rods, there were no significant differences in the activity y of standards of equal Co content due to different sizes of standards with the measuring geom- etries applied.

Results were therefore tied together to form sets of equal Co content (see Table 11). The Co content m, number n of standards per set, mean relative signal intensity y, and relative standard deviation Ay/y of standards are given in Table 11.

Due to the constant relative standard deviation Ay/y in the mass range from 20 to 6000 ng, unweighed linear regression of the logarithms of Co masses m and relative signal intensities y had to be performed,49 instead of the data themselves, leading to logy = f l log m + f,,. The slope fl of the regression line is 1.0023 f 0.0005, and the relative residual scatter above the calibration line is 2.2%. From statistical evaluation, a linear calibration y = blm was derived with bl = 0.9985 f 0.0014 ng-l and a correlation coefficient r = 0.999 993. The environmental background was measured to be 7 X lo9 counts/s. The absolute detection limit, mL (Table 111), was estimated to 1.2 pg of Co.

Analysis. The surface decontamination procedure resulted in an erosion of (6-30) X 1O'O atoms/cm2 of Co from the blank samples, as averaged for the total surface (cf. ref 42, Table 111, and the discussion given there). From the implanted Si surfaces, it resulted in an erosion of the following.

Co-L1: 20.5 f 3 nm of Si (with corresponding relative loss AD of the total implanted Co dose D, estimated to aO/D less than 1.2%; compared to a measured surface decontamination of 9.4 X 1O'O atoms/cm2, referring to the implanted side only).

Co-L2: 60 f 3 nm of Si (with a corresponding relative loss of overall Co, including any contamination, measured to AD/D = 0.38%).

Thus, care was taken to suppress blank considerably, to improve detection limit (cf. ref 42), but not to deteriorate the complete recovery of the ion-implanted Co.

Results from individual samples, as obtained from cali- bration and area determination, are given in Table 111.

Compared to the implanted dose as determined by ion current measurements, a significant systematic deviation

is found on the average within the dose range D from 10l2 to 5 X 10l6 Co ions/cm2 (subscripts refer to methods of deter- mination). This is in complete agreement with results from XRFA, FAAS, and RBS, where analogous systematic devia- tions were AreJIXRFA = -0.106 f 0.025, 4e1DFAAS = -0.056 f 0.062, and A&RBS = -0.087 f 0.037 in the dose range from 5 x 1015 to 1.6 x 1017 Co ions/cm2 as published r e ~ e n t l y . ~ ~ , ~ Due to this complete agreement, the DICM values were re- garded to be in systematic error of -10% on the average.

Detection limits of further elements were determined using the single comparator standardization technique (Gent's ko

A r e i D N u = ( D N u - DICM)/DICM = -0.104 f 0.028

1104 ANALYTICAL CHEMISTRY, VOL. 64, NO. 10, MAY 15, 1992

method60*61). Estimated figures of merit for 56 elements are given in Table I for (a) the experimental conditions applied in this work, in column 6 and 7, and (b) in column 8, as recalculated from bulk impurity data by INAA of ultra- high-purity silicon, given by Verheijke et al.40,42 and Bottger et

DISCUSSION Range of Elements. The elements listed in Table I are

confied to those where nuclides are available which (a) can be economically implanted, i.e., are of reasonable natural abundance (see Table I column 3), and (b) which form acti- vation products by reactor neutron irradiation of the im- planted nuclei that can be measured with good sensitivity by y-spectrometry (see Table I column 61, after a thorough surface cleaning, considering also (c) the half-life T = 2.6 h of the high matrix activity 31Si. We therefore dismissed (a) nuclei of low natural abundance a <0.1 (6 exceptions due to better detection limits), and nuclear reactions yielding prod- ucts of (b) poor resulting sensitivity per counting interval s/tm < 10-4 counta/s per 10l2 implanted atoms (viz. leas than loo00 counts during t, = 2 days from a 6-cm2 sample of dose D = 1014 ions/cm2), or (c) short half-lifes below 2 h.

Not considered in Table I are nuclides that can be activated via less commonly applied nuclear activation processes such as charged particle activation analysis (CPAA),62 y-photon activation analysis,63~" fast (generator) neutron activation analpis (FNAA),55B and prompby neutron activation analysis (PGNAA) .30,67

Detection Limit. From Table I, detection limits of the dose DL, are compared between this work and data based on results published earlier by Verheijke et a1140p42 and Bottger et al.4' Differences in the detection limits (cf. ref 58) obtained in different laboratories are governed mainly by the impurity level and sample size of the Si wafers analyzed, the irradiation time, irradiation flux, and other irradiation conditions, the chemical treatment of the samples, and the y-spectrometric conditions and equipment used (cf. refs 36, 42, 59, and 60).

The objective of the present procedure was not to obtain the ultimate detection limits for a variety of elements, but to yield a relative accuracy of 1-2% for the determination of cobalt to obtain reasonable good calibration of the implanted silicon wafer material. We did not use ultra-high-purity material, but merely ion-implanted material of fairly high starting purity. We emphasized measures to optimize accuracy and precision (cf. refs 31, 61, and 62), by careful control of surface contamination, by using a spectrometer of high effi- ciency, and by long measuring time duration t, on the one hand, but restricting, on the other hand, the maximum sample area and the maximum total countrate (see measurements) for the benefit of precision, thus simultaneously sacrificing sensitivity. The rather low neutron flux of 9 X 10l2 n cm-2 s-l was applied, because the respective irradiation position was well characterized for high-accuracy INAA work from earlier ~tudies .3~8~ The lower detection limits obtained by Verhejke et a l . 4 0 s 4 2 and Bottger et represented in Table I, are correspondingly due to the ultra-high-purity materials used, larger sample sizes, and higher neutron fluxes. Those data are included to elucidate what fwes of merit can be obtained experimentally, under conditions favorable for ultra-trace analysis, and also to indicate where the useful range of thin- film analytical applications can be pushed to, with the dif- ferent nuclides mentioned in Table I.

Advantages of INAA. In comparison to the XRFA, FAAS, and RBS results mentioned (see refs 15, 43, and 64; however, also ref 5) , the detection limit (by a factor of 5 X lo4) as well as the precision and accuracy are strongly im- proved with the data given here. The detection limit obtained with INAA is 1.2 pg of Co, corresponding to DL = 6 X lo9

atoms/cm2. It could be even improved, if necessary, by simply applying higher neutron flux irradiation, since it is not limited, in the present case, by sample blanks, but by y-background radiation from the detector environment, on condition that sample surface contamination is carefully controlled (see Control of Surface Contamination). Improving the detection limits and the dynamic range was important (a) to study any possible systematic effects or errors as a function of implanted ion dose, and (b) to make available calibration data of im- planted doses less than 10l5 atoms/cm2 for SIMS depth profiling with high elemental sensitivity.

Obviously, more simply operated methods like XRFA are to be preferred against NAA, as long as their detection limits and dynamic ranges, as well as their accuracies obtained, will suffice for the individual analytical task. Moreover, all bulk analytical methods, in general, will not give any information about the depth distribution in the sample. Methods that rely, even in its quantitative evaluation, on well-established and surveyable physical principles, like RBS, are therefore of fundamental importance for calibration work on depth pro- filing reference material^.'^-^^^ Keeping in mind, however, that they are not a priori inaffected by any systematic errors, they are often leas precise and accurate, and sometimes subject to more severe matrix effects.

Considering the widespread prejudice against nuclear methods, we emphasize that the expenditure of effort and sophistication in this calibration work is solely and completely aimed at a high level of accuracy over a wide dynamic range down to very low elemental contents, and not simply a t all coupled to the use of a nuclear method or the processing of active material (see also refs 31,61, and 62). The predominant advantage of INAA in this application, besides the inherent high sensitivity, is in its outstanding robustness (a) against problems of sample contamination during the analytical procedure, since samples are processed after being activated, and (b) against matrix effects which are often found in com- peting analytical methods. Thus, the work presented here is based on the condition that a small radiochemical laboratory and y-spectrometer (typical costs in the range of $1OOOOO, similar to other analytical spectrometric equipment) are available, the irradiations can be run on a typical nuclear reactor (offering moderate neutron flux of 1013 n cm-2 s-l), and sufficient time is at disposal for irradiation, cooling of the samples, and measurement of even low activities with suffi- cient accuracy. It is felt that the case of calibration of ref- erence materials is generally compatible with these require- ments. So, the established method of INAA can add an im- portant piece of analytical reliability to the benefit of thin-film depth profiling.

CONCLUSION Before use, reference samples for depth profiling analy-

sis6g70 should be subjected to a completely independent bulk analytical determination by a powerful trace analytical technique with high accuracy. Activation analysis in general, and especially NAA, offers great potential for calibration of the total elemental quantity of a variety of elements even on extreme trace levels. The conditions applied have been found very suitable for highly accurate cobalt determination. The detection limit of ca. 1 pg of cobalt was quite adequate, and sufficient reserve has been demonstrated to exist for consid- erable further improvement, if desirable.

ACKNOWLEDGMENT M. L. Verheijke kindly provided additional details on his

INAA work concerning the detection limits referenced in Table I. Thanks are due to the staff of the FRJ-2 reactor at Julich for the careful execution of the irradiation. Parta of this work were financially supported by "Ministerium fiir

ANALYTICAL CHEMISTRY, VOL. 64, NO. 10, MAY 15, 1992 1105

Wissenschaft und Forschung des Landes Nordrhein- Westfalen” and the German “Bundesministerium fiir For- schung und Technologie”. Performing the reactor irradiation free of charge by the “Forschungsanlage JiilichlD”, is grate- fully acknowledged. Registry No. Na, 744023-5; Mg, 1439-95-4; Al, 7429-90-5; Sc,

1440-20-2; Mn, 1439-96-5; Fe, 1439-89-6; Co, 1440-48-4; Nii 1440-02-0; Cu, 1440-50-8; Zn, 7440-66-6; Ga, 1440-55-3; Ge, 1440-56-4; As, 1440-38-2; Se, 1182-49-2; Br, 1726-95-6; Kr, 7439- 90-9; Rb, 1440-17-1; Sr, 1440-24-6; Y, 1440-65-5; Zr, 1440-67-7;

1440-43-9; In, 1440-74-6; Sn, 1440-31-5; Sb, 1440-36-0; Te, 13494-80-9; Xe, 1440-63-3; Cs, 1440-46-2; La, 7439-91-0; Ce, 1440-45-1; Pr, 1440-10-0; Nd, 1440-00-8; Sm, 1440-19-9; Eu,

7440-60-0; Er, 1440-52-0; Tm, 7440-30-4; Yb, 1440-64-4; Lu, 7439-94-3; Hf, 1440-58-6; Ta, 1440-25-1; W, 7440-33-1; Re, 1440-15-5; Os, 1440-04-2; Ir, 7439-88-5; Pt, 1440-06-4; Au, 1440- 51-5; Hg, 1439-91-6; Th, 1440-29-1; U, 7440-61-1; Si, 1440-21-3.

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1440-53-1; Gd, 7440-54-2; Tb, 1440-21-9; Dy, 7429-91-6; Ho,

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RECEIVED for review October 8,1991. Accepted February 13, 1992.